In a gas turbine engine, operational efficiency generally increases as the temperature of the combustion stream increases. Higher combustion stream temperatures, however, may produce higher levels of nitrogen oxides (NOx) and other types of emissions. These emissions may be subject to both federal and state regulation in the United States and also subject to similar regulations abroad. A balancing act thus exists between operating a gas turbine in an efficient temperature range while also ensuring that the output of NOx and other types of regulated emissions remain below mandated levels. Further complicating this balance, non-stoichiometric combustion may produce nitrogen oxides at the high temperatures as described above. Reducing combustion temperatures, however, may result in higher carbon monoxide (CO) emission levels because of the incomplete oxidation of the fuel. Carbon monoxide also is a regulated emission.
Recently, various types of exhaust gas recirculation (EGR) techniques, and more particularly stoichiometric exhaust gas recirculation (SEGR) techniques, have been useful in reducing overall emissions of nitrous oxides and carbon monoxide. Turbines using stoichiometric exhaust gas recirculation techniques, however, may require a significant increase in the amount of air extracted from the combustor, about thirty percent (30%) or more depending upon the flow needed to replace the oxidizer such as air or oxygen. Such a significant extraction, however, may impact at least on overall combustor cooling and combustion temperatures therein as well as the overall lifetime of the combustor components.
There is thus a desire for an improved combustor design for the use with a stoichiometric exhaust gas recirculation turbine system. Preferably such a design would permit adequate air extraction to the turbine while maintaining adequate combustor cooling and combustor temperatures for improved efficiency and overall component lifetime.